1. Field of the Invention
The present invention relates to apparatus for compensating for a quantity of light and, more particularly, to a light amount compensation apparatus applicable to a system which uses a laser beam modulated with information signals for recording information on a photosensitive material.
2. Description of the Prior Art
In an information recording system of the type described above, the recording laser beam has to be stable, and especially stable when image information to be recorded requires relatively high-quality images such as diagnostic images and remote sensing images. The problem encountered in this respect is that the amount of light emitted from a laser fluctuates with the lapse of time and contains noise and, additionally, the performance of the laser per se deteriorates as the time passes by.
Various kinds of photosensitive materials are used according to the properties or applications of the image information to be recorded. The primary requisite is that the amount of exposure to the laser beam be controlled to match with the specific sensitivity (ASA sensitivity) of the photosensitive or radiation-sensitive material used, thereby allowing an optimum portion, or latitude, of the gradation reproduction characteristic curve particular to the photosensitive material.
One approach heretofore proposed for such light quantity compensation is to adjust the optical density of a neutral density (ND) filter which is disposed in the output optical path of the light source, i.e. laser. However, this allows the filter density to be adjusted only in a stepwise manner and not exactly linearly along with fluctuations of the laser characteristics. Another problem inherent in this approach is that the filter has to be replaced manually requiring work which is time- and labor-consuming.